The invention relates to a connection element.
Connection elements of this type are used in integrated circuits in order, for example, to activate redundant memory cells in dynamic random access memories (DRAMs). Furthermore, connection elements of this type can be used in order to write identifiers to chips or in order to change functionalities of chips in a targeted manner. In particular, connection elements of this type can be used as programmable connection elements in the field of FPGAs.
In this case, the connection elements can be configured as fuse circuits or antifuse circuits. A fuse circuit has a structure via which a current flows as long as the structure is intact. The structure can be destroyed by irradiation with laser light or by an application of a voltage, with the result that current can then no longer flow.
A connection element configured as an antifuse circuit typically has a layer structure having an insulating layerxe2x80x94such as, for example, a dielectric layerxe2x80x94which is disposed between two conductive layers.
With the insulating layer intact, no current flows via the connection element. Only when a predetermined electric voltage is applied to the connection element is the dielectric layer destroyed, with the result that a current can then flow.
The voltage is applied via contacts, interconnects or similar structures which are led to the conductive layers of the layer structure of the connection element. In known connection elements of this type, the conductive layers of the layer structure are composed of polysilicon or amorphous silicon. A conductive structure composed of tungsten is led to at least one of the layers. The conductive structure may be formed, in particular, by an interconnect on whose surface the layer structure is applied.
Configurations of this type have the disadvantage that high deposition temperatures are required for the deposition of the tungsten forming the interconnect and also of the polysilicon layers. As a result of this, the tungsten which forms the interconnect reacts with the adjoining polysilicon layer to form WSix, as a result of which an undesirably rough interface between the interconnect and the conductive layer is obtained.
As a result, there is the risk that the polysilicon layer deposited on the tungsten will have a greatly inhomogeneous layer thickness, as a result of which electrical breakdowns can arise which greatly impair the functionality of the connection element.
Finally, it is disadvantageous that polysilicon has a comparatively low conductivity, so that relatively high voltages must be applied to the connection element in order to destroy the insulating layer.
U.S. Pat. No. 5,602,053 describes a method for fabricating an antifuse structure. The antifuse structure is disposed between two conductive contacts. Each contact has a lateral barrier layer made of titanium nitride, titanium tungsten or tantalum nitride. A layer made of aluminum, copper, tungsten or silver is deposited within the barrier layer. A layer structure formed from four layers lying one above the other is applied as the antifuse structure to the upper interface of such a contact. The first layer is composed of silicon nitride and bears on the interface of the contact. A layer made of amorphous silicon is applied to the silicon nitride layer, and a layer made of silicon nitride is in turn applied to the layer made of amorphous silicon. A further layer made of amorphous silicon is applied on the layer made of silicon nitride. Finally, the top side of the further layer made of amorphous silicon is adjoined by the second contact.
It is accordingly an object of the invention to provide a connection element which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has a high reliability in conjunction with the smallest possible topography.
With the foregoing and other objects in view there is provided, in accordance with the invention, a connection element formed in an integrated circuit. The connection element contains a substrate, an interconnect disposed on the substrate and formed of tungsten, an insulating layer disposed on the interconnect, and a silicon layer disposed on the insulating layer and formed of a polycrystalline doped silicon. The insulating layer can be destroyed by an application of a predetermined voltage to produce a conductive connection between the interconnect and the silicon layer.
The connection element according to the invention has a layer structure disposed between two conductive structures. The layer structure contains an insulating layer, which can be destroyed by application of an electrical voltage, and a silicon layer. The silicon layer is preferably composed of polysilicon or Si3N4. In principle, the silicon layer may also be composed of amorphous silicon.
The insulating layer adjoins a conductive structure composed of tungsten. In an advantageous embodiment, the insulating layer bears directly on the surface of the interconnect composed of tungsten.
Thus, the basic concept of the invention is that, in contrast to conventional layer structures, the insulating layer is not disposed between two silicon layers. Rather, the insulating layer bears directly on the interconnect composed of tungsten.
An essential advantage of the configuration is that, at the interface between the conductive structure composed of tungsten and the layer structure, interface reactions between the polysilicon and the tungsten which lead to roughening of the interface are avoided.
Thus, when the insulating layer is applied to the surface of the conductive structure composed of tungsten, in particular the interconnect composed of tungsten, a homogeneous layer thickness of the insulating layer is obtained.
As a result, the risk of electrical breakdowns in the region of the insulating layer is largely avoided, so that high reliability of the connection element is obtained.
It is particularly advantageous, moreover, that the deposition of the insulating layer on the tungsten can be carried out at high deposition temperatures since, at the interface, there is now no risk of a reaction of the tungsten with the silicon to form WSix. On account of this, furnace deposition processes can be used for fabricating the dielectric layer, thereby obtaining particularly controlled process management. In particular, this results in particularly uniform layer thicknesses of the insulating layer.
A further essential advantage of the connection element according to the invention is that the layer structure has only two layers. This results in a particularly small topography of the layer structure. Moreover, the latter can be fabricated simply and in a small number of method steps.
Furthermore, it is advantageous that, as a result of the insulating layer being directly linked to the conductive structure, low voltages already suffice to destroy the insulating layer.
In an advantageous embodiment of the invention, the second conductive structure is formed by a contact made of tungsten. In order to fabricate the contact, a contact hole is etched into an oxide layer as far as the silicon layer of the layer structure, the silicon layer adjoins the underside of the oxide layer. The silicon layer, which is preferably composed of polysilicon, serves as an etching stop for the etching process.
In accordance with an added feature of the invention, the interconnect has a top side and the insulating layer is applied to the top side of the interconnect by a furnace deposition process.
In accordance with an additional feature of the invention, the furnace deposition process is a low pressure chemical vapor deposition method.
In accordance with another feature of the invention, a contact is disposed on the silicon layer and a further interconnect is connected to the contact. The silicon layer is connected to the further interconnect through the contact.
In accordance with a further feature of the invention, an oxide layer having a contact hole etched therein is provided. The contact is formed by a tungsten layer deposited in the contact hole. The silicon layer forms an etching stop layer for an etching of the contact hole.
In accordance with a concomitant feature of the invention, the further interconnect contains aluminum.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a connection element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.